EP - Hello Black Blur!

[mulder85]

Hello all,

I've got a Skywatcher 8" Dob, Canon 1200D, and the usual accessories for EP(T-Ring, EP adapter). I have tried using a 25mm eyepiece on it and the results were ok, but the amount of magnification was no more than Prime Focus with a 2x Barlow. So I tried using my 10mm eyepiece. The result is that I don't seem to be able to see anything when I do that. Eternal blackness and blurriness.  Can't focus on anything. In fact I just can't see anything to focus on! I suppose the issue has been mentioned before, but I was wondering if you guys know of this issue and could possibly provide solutions. What is the problem? Do I need to use on of those variable length EP adapters instead(mine is fixed) ? Do I need to add a Barlow in the middle? Any advice would be appreciated. Thanks!

 

Alex

 

[Cornelius Varley]

From the description I assume you are trying eyepiece projection photography ? The trick with this technique is not to use a high powered eyepiece, ideally something around 20mm is better. Take a look at Anne Diittmer's website .The technique employed to take the images is ep projection, the site is a little old and she uses (used) film cameras to take the images but the concept is still valid. The Nuts and Bolts section gives some mathematical equations to determine the image scale and focal ratio .  The telescope and eyepiece focal lengths and the distance between the eyepiece and camera have an affect on the apparent magnification and focal ratio. At 10mm the apparent ratio might be very high and result in a very dark, blurred image on the camera.

[mulder85]

Thanks for the reply Cornelius. I will surely have a look at those. The way you put it, it seems that the root cause is that the 10mm eyepiece increases the f to huge numbers, near pitch black, and that's why I can't see anything? In that case, Would buying a an eyepiece of between 15-20mm be better? If I buy a variable EP adapter instead and still use the 10mm eyepiece, can I achieve a good result by decreasing the magnification via the variable valve?

 

Thanks,

Alex

[Cornelius Varley]

1 hour ago, mulder85 said:

Thanks for the reply Cornelius. I will surely have a look at those. The way you put it, it seems that the root cause is that the 10mm eyepiece increases the f to huge numbers, near pitch black, and that's why I can't see anything? In that case, Would buying a an eyepiece of between 15-20mm be better? If I buy a variable EP adapter instead and still use the 10mm eyepiece, can I achieve a good result by decreasing the magnification via the variable valve?

 

Thanks,

Alex

The 10mm eyepiece supplied with skywatcher telescopes isn't very good and trying to image with it will probably show up its defects even more. Using the adapter with the 10mm will still cause imaging problems. The adapter works by increasing the distance between the eyepiece and focal plane, the greater the distance the greater the apparent magnification and so an increase in apparent focal ratio and if you are already getting unsatisfactory results with the 10mm you will not improve things with the variable adapter.  An eyepiece in the range mentioned is a good start. The eyepiece also needs to produce a flat field from edge to edge so as to avoid field curvature where the image is sharp at the centre and gets progressively blurred towards the edge of the glass.

 

[mulder85]

2 hours ago, Cornelius Varley said:

The 10mm eyepiece supplied with skywatcher telescopes isn't very good and trying to image with it will probably show up its defects even more. Using the adapter with the 10mm will still cause imaging problems. The adapter works by increasing the distance between the eyepiece and focal plane, the greater the distance the greater the apparent magnification and so an increase in apparent focal ratio and if you are already getting unsatisfactory results with the 10mm you will not improve things with the variable adapter.  An eyepiece in the range mentioned is a good start. The eyepiece also needs to produce a flat field from edge to edge so as to avoid field curvature where the image is sharp at the centre and gets progressively blurred towards the edge of the glass.

 

Thanks again. Now I got it right.  So it's the eyepiece. Have you got a specific eyepiece to recommend on the 15-20mm range, that would work well with my EP setup, without breaking the bank?

 

Thanks,

Alex

[Cornelius Varley]

1 hour ago, mulder85 said:

Thanks again. Now I got it right.  So it's the eyepiece. Have you got a specific eyepiece to recommend on the 15-20mm range, that would work well with my EP setup, without breaking the bank?

 

Thanks,

Alex

The Televue range of Plossls are quite good. I have a couple of these, I don't use them for imaging through, just visual and are nice. The Baader Hyperion range, including the zoom, have a built in 43mm thread which makes it easy to connect to a camera body using the Baader 43mm/T thread adapter. These are about £99 and £185 for the zoom so not so cheap. A lot of modern eyepiece designs these days use much larger barrels than can be used with the variable ep adapter.  

[Cornelius Varley]

Some you should also take into consideration when imaging is the apparent focal ratio. As the magnification increase so will the focal ratio which will also make the image darker (this you have already noticed). The darker image might also need a longer exposure and without a tracking mount exposure times will need to be short to avoid blurring.

[mulder85]

Thanks for the valuable info again. Since I am mostly interested in planetary viewing and imaging, I wouldn't want to invest a lot of money on High mm eyepieces.  therefore I was thinking of getting a 15mm Plossl. Considering that 25mm worked for EP and 10mm didn't, do you think a 15mm could solve the darkness issue, or do I have to go even higher at 20mm? That's cutting it pretty close to my already existing 25mm so the magnification would not be as exciting as I'd want.  In any case, on a more general note, do you think that that the Original Skywatcher 25 and 10mm eyepieces supplied with the telescope are useless? Should I do my best to replace them?

 

Alex